1. Technical Field
The present invention relates to backlight modules typically used in liquid crystal displays (LCDs) and, more particularly, to backlight modules with highly uniform illumination.
2. Description of the Related Art
Color LCD devices have been widely used in various applications, such as in portable personal computers, LCD televisions, video built-in type LCDs, etc. A conventional LCD device mainly includes a backlight module and a liquid crystal panel. An under-lighting system or an edge-lighting system is used as the backlight module. In an under-lighting system, a light source is disposed under a diffusion plate, and the diffusion plate is disposed under the liquid crystal panel. In an edge-lighting system, a light source is disposed at a side surface of a light guide plate (LGP), and the LGP is disposed under the liquid crystal panel.
Typically, an edge-lighting system includes an LGP and a light source. The LGP is formed from a planar transparent member, such as an acrylic resin plate or the like. Light beams emitted from the light source are transmitted through a side surface (i.e., light incident surface) of the LGP into the LGP. Most of the incident light beams are internally reflected in the LGP between a light emission surface and an opposite bottom surface of the LGP and are then transmitted more or less uniformly out through the light emission surface of the LGP. A plurality of light diffusion dots, having a light scattering function, are advantageously formed on the bottom surface, in order to increase the uniformity of illumination of the backlight module. The light source is usually at least one linear source, such as a cold cathode fluorescent lamp (CCFL), or at least one point source, such as a light emitting diode (LED).
The configuration of the diffusion dots is key to good optical performance of the LGP. Thus, various configurations of diffusion dots of LGPs have been devised recently. FIGS. 9 and 10 show a conventional backlight module including an LGP 22, a CCFL 21, a reflection sheet 25, a prism sheet 27, and three side reflectors 29 (only one shown). The LGP 22 has a light incident surface 223, a bottom surface 222, an emission surface 221, and three side surfaces 224, 225. The CCFL 21 is arranged adjacent to the light incident surface 223. The reflection sheet 25 is placed under the bottom surface 222. The prism sheet 27 is set above the emission surface 221. One of the side reflectors 29 is arranged adjacent to the side surface 224. The other two side reflectors 29 are aligned respectively adjacent to their two corresponding side surfaces 225. A plurality of diffusion dots 26 are provided on the bottom surface 222, generally in a regular array of rows and columns. The diffusion dots 26 are ordered in a manner such that sizes thereof in a first main region A of the bottom surface 222 increase with increasing distance away from the CCFL 21, and sizes thereof in a second region B of the bottom surface 222 adjacent to the side surface 224 are the same. The sizes of the diffusion dots 26 in region B are substantially the same as a size of those diffusion dots 26 in region A that are adjacent to region B. The diffusion dots 26 in any column of the array parallel to the CCFL 21 have a similar size.
Generally, CCFL 21 light intensity in region A decreases with increasing distance away from the CCFL 21. Thus, the configuration of the diffusion dots 26 in region A can increase the uniformity of illumination on the emission surface 221 of the LGP 22, because intensity of light beams emitted from the emission surface 221 is substantially proportional to the sizes of the corresponding diffusion dots 26.
However, illumination in both regions A and B is uneven. One reason for this is because light beams are reflected by the side reflector 29 from region A back into region B, and the columns of the diffusion dots 26 in region B are spaced different respective distances from the side reflector 29. That is, the diffusion dots 26 in respective different columns in region B receive light beams having different intensities. Therefore, light beams do not emit uniformly from the part of the emission surface 221 corresponding to region B. Another reason is that the two side reflectors 29 that are adjacent to the two side surfaces 225 have a similar effect to the above-described operation of the side reflector 29 that is distal from region A. This contribution by these side reflectors 29 results in further uneven illumination between the side surfaces 225, in both regions A and B. Therefore, light beams do not emit uniformly from the part of the emission surface 221 corresponding to both regions A and B (i.e., the entire emission surface 221 of the LGP 22). In summary, respective distributions of the diffusion dots 26 in regions A and B result in non-uniform illumination over the whole emission surface 221 of the LGP 22.
Furthermore, if the CCFL 21 is replaced by a series of point sources such as LEDs, the uniformity of illumination of the backlight module is generally unsatisfactory. That is, the limited lighting characteristics of the LEDs result in a plurality of darker areas, generally between adjacent LEDs, being created in the LGP 22. In conclusion, it is very problematic to provide even illumination throughout the entire emission surface 221 of the LGP 22.
What is needed, therefore, is a backlight module that overcomes the above-mentioned problems and thereby provide more even illumination throughout the entire emission surface of a given LGP.